Dimers which are often used in order to control the molecular weight in solution polymerizations are e.g. the α-methyl styrene dimer (CAS 6144-04-3 or 6362-80-7) and MMA (methyl methacrylate) dimer (CAS No. 71674-93-6 or 28261-32-7). For control of molecular weight in emulsion polymerizations only the α-methylstyrene dimer has been used by Japan Synthetic Rubber Co (U.S. Pat. No. 5,637,644 and U.S. Pat. No. 5,444,118) until now.
In JP-A 11-292907 e.g. Sekisui was using the α-methylstyrene dimer in a surfactant free emulsion polymerization of styrene which was resulting in the synthesis of a polymer latex having large particle sizes.
Similar to the use of these dimers in emulsion polymerization, macromers can be used. A useful macromer is first prepared by making use of a solution polymerization step. After isolation of this macromer, it is further used in an emulsion polymerization step. This technique has e.g. been applied by Du Pont (in WO 99/42505) and by Rodia Chimie (see WO 99/57167). That polymerization method however implies that two steps are required: a first solution polymerization step and, subsequently, an emulsion polymerization step wherein use is made of the macromer prepared in the first solution polymerization step mentioned hereinbefore.
Control on particle sizes in emulsion polymerization can be done by controlling preparation parameters as amount, type and mode of addition of initiator and surfactant. Addition of traditional chain transfer agents like mercaptans may also have an influence on particle size, but in this case only small changes in particle size are obtained. When making use of lauryl mercaptan it has often been found that the particle size slightly increases because of the presence of more droplet nucleation as the lauryl mercaptan dissolves in the monomer phase and as it allows a higher degree of droplet nucleation, as in mini-emulsion polymerizations, wherein cetylalcohol is used as co-surfactant. In case of the novel chain transfer agents, such as dimers and cobalt complexes, particle size can decrease drastically. A higher number of particles is obtained, presumably due to a higher radical flux. Exit of radicals may occur more frequently, which gives re-entry in another micelle (micelle nucleation) or extra initiation of monomers in the aqueous phase (homogeneous nucleation). In case of the use of dimers, radical addition fragmentation gives a more water soluble radical which can enter in the aqueous phase. The employed dimer can be designed in order to have good copolymerization parameters with the monomers used and in order to obtain radicals with enough water solubility after fragmentation. Also the water solubility of the cobalt complexes can be adjusted.
Introducing dimeric compounds by addition fragmentation transfer is expected to give macromers with unsaturated endgroups. Said macromers might be used for further polymerization, resulting in the synthesis of graft- or block copolymers. Besides the α-methylstyrene dimer and the MMA dimer also functional dimers could be used, resulting in end-functional telechelics. So in U.S. Pat. No. 5,264,530 an improved method of free radical polymerization in an aqueous system, wherein the method employs a macromonomer mixture, having terminal ethylenic unsaturatation, as a chain transfer agent under aqueous conditions. Such macromonomers are advantageous for controlling the molecular weight of polymers or copolymers produced therewith but molecular weight reduction has little or no effect on mean particle size as has been established therein. Although the lowest particle size obtained is about 80 nm, a reduction percentage of only 7% versus the comparative example can be reached. The limited effect upon particle size in U.S. Pat. No. 5,264,530 can be explained as follows.
1) The influence of dimers on particle size will be much more pronounced at low surfactant concentrations. If a surfactant concentration is used far above its CMC (critical micelle concentration: micellar nucleation will be more decisive than homogeneous nucleation for the result obtained. Employed dimers will be more effective in reducing the particle size in case that particle nucleation occurs via homogeneous nucleation. In U.S. Pat. No. 5,264,530 a combination of two anionic surfactants is used [Trem LF 40 (allyl dodecyl sulfosuccinate sodium salt) and Dupanol WAOI (sodium lauryl sulfate)]. The initial surfactant concentration is about 4.6 gram/liter TREM LF40+3,9 gram/liter Dupanol WAOI which result in a total amount of 8.5 gram/liter of anionic surfactant. The CMC of Dupanol WAOI is about 2.05 gram/liter. So in case of the emulsion copolymerisation, described in the US-A reference, the surfactant concentration is much higher than the CMC, and so micellar nucleation will influence particle size more pronounced.
2) If a mixture of oligomers is used, larger particle sizes might be obtained due to a lower solubility of the longer oligomers in water. When longer oligomeric radicals are present in a latex particle, re-entry in the aqueous phase is difficult due to the limited water solubility. As a comparison when pure dimeric compounds are used the addition-fragmentation reaction yields a monomer radical, which easily can give a re-entry in the aqueous phase. Furthermore, the presence of longer oligomers gives rise to the formation of block- or graft-copolymers, which may act as in-situ formed surfactants and consequently alter the particle nucleation mechanism.
3) As described by Caterine L. Moad in Macromolecules, 1996, volume 29, page 7717–7726 a pure dimer behaves much more different than the trimer or higher macromonomers, with respect to chain transfer activity, kinetics and mechanism of chain transfer. The rate determining step, i.e. the addition of the double bond and formation of the radical intermediate is mainly controlled by steric effects. In case of an adduct of the dimer the steric repulsion is much less. Differences in kinetics, chain transfer mechanism and steric hinderance of pure dimers in comparison with a mixture of oligomers might influence particle nucleation, and consequently also particle size.
Cobalt complexes are well-know as chain-transfer agent used in order to control polymer molecular weights, but use of cobalt complexes, even in combination with unpurified monomers (containing still some inhibitor), thereby causing a significant particle size decrease, hardly affects molecular weights of polymers thus formed.
Control of molecular weight and particle size now is particularly important in applications, where e.g. sharp-transition melting or latex coalescence is used. Also in applications where transparency and where the specific surface area is important, research to provide control of particle size is crucial. Furthermore smaller particle sizes can facilitate film formation. In some cases this might lead to the situation that less plasticizer is required to give sufficient film formation using dispersions or latices with a high glass transition temperature (Tg) or a high minimal film forming temperature (MFT).
So it is e.g. well-known that is difficult to obtain small particle sizes in emulsion polymerization, without making use of high surfactant or initiator concentrations.
High concentrations of surfactant or initiator can provide undesired polymer mixtures, e.g. when polymer latex particles are used in ink-jet applications or as surface sizing agents. In that case said high concentration of surfactant may lead to bleeding of ink and to an inferior image quality.